Regulated filament supply



Sept. 2, 1958 e. s. KAN EIAL REGULATED FILAMENT SUPPLY Filed NOV. 5,1954 I 54 g I I ZNVENTORS WWI Ii V ZTOPA/EK? l flmiard United StatesPatent Cfiice 2,85ll,15?5 Patented Sept. 2, 1958 REGULATED FILAIVZENTSUPPLY George S. Kan and Bernard M. Oliver, Palo Alto, Calif., assignorsto Hewlett=Packard Company, Pain Alto, Calif a corporation of CaliforniaApplication November 5, 1954, Serial No. 467,122

7 Claims. (Cl. 315-97) This invention relates to an electronic networkfor supplying regulated filament power.

It is well known that variations in a filament power vary the electronemission, and therefore, the current conducted by a vacuum tube. Thisaffects the performance and characteristics of various electronicnetworks which incorporate a filament power supply for the vacuum tubes.For example, in an oscillator network, variations in the filament supplyafiect the frequency of operation by influencing the alternating currentflow through the tank circuit by modifying the plate cathode impedanceof the vacuum tube or tubes employed. In electronic amplifying networkssuch changes vary the amplification for a given plate voltage.

It is conventional practice for electronic networks to include ballasttubes for regulating the filament power, such tubes being placed inseries with the filament supply. As the current tends to vary, thefilament temperature and resistance vary to effect current regulation.Such ballast tubes are subject to certain disadvantages. Normally thefilament is operated within a temperature range at which it tends tobecome brittle. Therefore, the filament is susceptible to breakage bymechanical shock or vibration. The efiiciency is relatively low becausesuch tubes insert a resistance in series with the current supply. Alsothe regulation is not as accurate or complete as is frequently desired.Special current regulating transformers have been used in place ofballast tubes, but they are not satisfactory for many types ofequipments, due to their weight, size and frequency limitations.

In general, it is an object of the present invention to provide anelectronic network having a novel regulated filament current supplyincorporated in the same. In particular, the present invention ischaracterized by freedom from injury due to mechanical shock orvibration, good efficiency and good regulation.

Additional objects and features of the invention will appear from thefollowing description in which the preferred embodiment has beendescribed in detail in conjunction with the accompanying drawings.

Referring to the drawings:

Figure l is a circuit diagram illustrating an electronic networkincorporating one embodiment of the present invention;

Figure 2 is a circuit diagram illustrating an electronic networkincorporating another embodiment of the present invention;

Figure 3 is a circuit diagram illustrating still another embodiment ofthe present invention.

The particular network illustrated in Figure 1 is a filament supplymeans for supplying a regulated current to the filaments (i. e. cathodeheater) of vacuum tubes of associated circuits. The network illustratedincludes two parts, namely, the part 11 which comprises the filamentsupply means, and the part 12 which shows a plurality 1 of tubes beingsupplied with regulated filament current. The current supply means 11 isa multivibrator of the vacuum tube type, making use of two vacuum tubes13 and 14, each having plate, cathode and control grid element. By wayof example, these tubes may consist of a single twin triode. Atransformer 16 serves to couple the multivibrator to the filamentcircuit of the associated circuit 12. One terminal of the primary oftransformer 16 is connected to the plate of tube 13, and the otherterminal to the plate of tube 14. The remaining elements of themultivibrator circuit are such as to cause it to function as a squarewave generator. Thus the cathodes of tubes 13 and 14 are grounded. Thecontrol grid of tube 13 is connected to ground through the resistor 22.Similarly, the control grid of tube 14 connects to ground throughresistor 24. Condenser 26 con nects between the plate of tube 14 and thegrid of tube 13. Condenser 27 is similarly connected between the plateof tube 13 and the grid of tube 14. The resistors 21 and 23 connect theplates to the positive terminal of the plate supply through line 19. Theplate current supply unit can be of conventional construction, andincludes conventional regulating means to maintain the plate voltagesubstantially constant.

As is well known to those familiar with construction and operation ofmultivibrators the two tubes 13 and 14 are alternately conducting andnonconducting, whereby the output of the transformer 16 is of squarewave form, except for modification of the waveforms by the inductivecharacter of the coupling. The frequency of operation of themultivibrator is determined by the time constants of the gridresistor-condenser arrangement employed. For the purpose of the presentinvention it is satisfactory to select a frequency of operation withinthe range of say, 50 to 5,000 C. P. S. It is apparent, of course, thatby proper selection of the circuit elements, it is possible to operatethe network at any desired frequency.

The part 12 shows the tubes 31 and 32 having filaments or cathodesheaters 33 and 34. These tubes might be, for example, in an oscillatornetwork, where variations in the filament supply affect the frequency ofoscillation, or in an electronic amplifying network where such changesvary the amplification for a given plate voltage. The filaments orcathode heaters are connected to the current supply means through thecircuit which include the transformer 16. Thus one side of the filamentsis grounded while the other is connected to one terminal of thesecondary of the transformer 16, in series with the adjustable resistor36. The other terminal of the transformer secondary is grounded.

Operation of the network illustrated in Figure 1 is as follows: Themultivibrator circuit 11 operates to produce a square waveform ofsubstantially constant amplitude, and this is employed to heat thefilament of tube 31. The amount of filament power supplied can beadjusted by varying the setting of resistor 36. During the conductionperiod of either tube 13 or 14, only a small portion of the B-supplyvoltage appears across the tube, the remainder is supplied to theexternal load. As a consequence, a given. percentage change in theVoltage drop across the tubes, due to changes in the tube parameters,results in a greatly reduced percentage change in the output voltage.Thus the regulation of the filament supply is inherently that of theregulated B-supply which supplies the plate voltage to themultivibrator. During conduction the tube drop is low and the tubelosses are at a minimum.

The operation and efficiency of the filament supply 11 is enhanced byemploying the circuit 11a of Figure 2. The plate resistors 21 and 23 arereplaced by transformer 46 having a center tapped primary 47. One end ofthe primary is connected to the plate of tube 13, the other to the plateof tube 14. I The center tap 47 is connected to the positive side of theplate supply voltage lead 19. The transformer 46 acts to transform theimpedance of the filaments 33 and 34 et al. of the associatedrtubes tobe suitable as a load in the plate circuit of thefilament supply-11a. Inthis manner only the power dissipated by the plate resistanceof the tubeand'the gridiresisto'rs is wasted, whereas in the circuit of Figure 1thegpower dissipated in resistors 21 and 23Was also wasted. Theremaining elements of the rnultivibrator circuit are such that theycause it to function as a square Wave generator. The cathode of tubes 13and '14 are grounded. The control grid of tube 1 3 is connected togroundthrough the resistors 48511. 49. Similarly,- the control grid oftube 14 is connected to ground through resistors 51 and 52. Condenser 37connects the plate of tube '14 to the common uncuanf'brresistbrs 48 and49. Condenser 38 connects between the plate 'oftub e 13 and the commonjunction or resistors. 51 and sz.

Resistors 48" ahd 51-opefate as follows: Assume that tube 14"has just"gene into conduction and tube 13 just out of conduction. The plate oftube 14 must drop from2E3 E to E' j wh'ei'e E, is the B su'pply voltageand E is the tube dro Thplate' o'f tube 13 must risefrom E to 2E ;'E, Topermit this rise, condenser 38 must be charged. If resistor 'srvr're'finrpie'sentthe charging would" benea'rly instantaneous. A highcurrent surge would flow 'from'the grid to cathode of tube 14 since thegrid to cathoder'esistance of this tube is'low. As' a result, bytransformer action, a high plate current will flow in tube' 14. Tosupply this current a higher plate voltage would be required. Because,of the higher plate current and voltage, ahigher dissipation loss wouldresult. By the addition of resistor 51, which limits the chargingcurrent, the charging action can take plate gradually over the firs tpart of the half cycle. The peak plate voltage and current requirementsare reduced, therefore, the plate dissipation is reduced.

Further, if the resistors 48 and 51 were not present the filament supplywould have a tendency to act as a blocking oscillator. The capacitor 38'charging instantaneously would cause a higher instantaneous platecurrent to flow in'tube '14 which would rapidly drop ofi as thecapacitor 33 became charged. Thus the input to the filaments 33 and 34'would be a pulse having relatively high amplitude and short duration.Resistor 51 prevents the capacitor from charging"instantaneously.Consequently'the input to f'the filaments is in creased because theinputis substantially of the square wave type. Resistor 48 serves the'same purpose'as res'i'stor' 51' when the tube 13 has gone" intoconduction.

As has beer'fdescribed' previously, the tubes 13 and 14 i R48R49 l 52 vR48+R49 R51+Rsz line-re s'If ss 4s= 51 and 49= 52- It an be shown thatthe energy stored in' the capacitors 37 and 38 is dissipated in theresistors and may be expressed as where E'is the voltage on' thecapacitor, C the capacitance and f the frequency of the niultivibrator.Thus if C can be reduced and the time constant maintained the efliciencyof the filament supply will be further increased.

The circuit of Figure 3 achieves this result. The time constant forcharging the capacitors 37 and 38 is now expressed as vibrator 11 canbe-selected such that the transformers 16 or 46 arerelatively light inweight and'of small; size.

'Further, thefrequency may be selected as not to affect.

the circuit for which the filament power is supplied.

The entire filament supply unit can be made relatively compact andlight-weight, and can'be incorporated as an integral part of'theequipment.

As previously explained the invention is applicable to varioustypesofelectronic networks. Thus, it 'caube used with amplifiers, countingcircuits and the like.

In one particular instance, the multivibrator circuit 11 shown in Figure3"was constructed as follows:

Tubes '13"a'nd"14 5687 Condenser '37 af 0.01 condens r; 38""; ,uf 0 .01Resistori49 kn '33 Resistor 52 kSZ I 33 Resistor 53 :kS2 f Resistor 54kQ- 68 The transformer 46 had a turns ratio of 2320:48. The

frequency'of the multivibrator 11 was 500 C. P; S.

For normal operation the average power supplied to the filament of tube31 was 3 watts. The plate eificiency of this apparatus was about 50percent. A variation of 30 percent in the voltage on the current supplylines supplying the filament of the multivibrator resultedin a filamentvoltage variation of the tube 31 of less than l percent.

We claim: 4

1. In an electronicnetwork of the type including a vacuum tube havingaheated cathode filament, and having the plate of a tube connected to adirect current source, a square wave-generator of the multivibratortype,

said'square wave generator including vacuum tubes having their platessupplied with direct current from said source, and means for couplingthe filament of the first named tube to the output of saidmultivibrator.

2. In an electronic network including at least one vacuum tube having acathode and having a plate connected to a source of direct current, asquare wave generator of the multivibrator type, said generatorincluding two vacuum tubes having their plates connected to said sourcethrough plate impedances, and means for coupling the output of themultivibratorto the filament of the first named tube.

3. Apparatus as in claim 2 in which the coupling means comprises atransformer having its primary connected between the plates of saidvacuum tubes, and the secondary connected to the filament of the firstnamed tube.

4. Apparatus as in claim 2 in which the coupling means and plateimpedances comprises a transformer having a center tapped primaryconnected between theplates of said two vacuum tubes with the center tapconnected to the said direct current source and providing plate currentto said tubes through the transformer impedance, and

7 thesecondary of said transformer connected to supply the filamentpower for said first named tube.

5 In an electronic network including at least one vacuum tube having acathode and having a plate connected to a source of direct current, asquare wave generator of the multivibrator type, said generatorincluding two vacuum tubes having their plates connected to said sourcethrough plate impedances and having current limiting resistors connectedin series with the plate to grid coupling, and means for coupling theoutput of the multivibrator to the filament of the first named tube.

6. Apparatus as in claim 5 in which the said coupling means and plateimpedances consists of a transformer having a divided primary, one-halfof the primary being in the plate circuit of one of the multivibratortubes and the other being in the plate circuit of the other tube.

7. In an electronic network including at least one vacuum tube having acathode and plate connected to a source of direct current, a square Wavegenerator of the multi-vibrator type, said generator including a pair ofvacuum tubes having at least plate, grid and cathode elements, anon-resonant transformer having a center tapped primary connectedbetween the plates, the center tap of said primary being connected tosaid source, a capacitor and current limiting resistor connected inseries and in the plate-to-grid path of each of said tubes, and thesecondary of said transformer connected to supply the filament power tosaid first named tube.

References Cited in the tile of this patent UNITED STATES PATENTS2,128,117 Braband Aug. 23, 1938 2,130,441 Wohlfarth et al Sept. 20, 19382,140,707 Lee Dec. 20, 1938 2,149,090 Wolfi Feb. 28, 1939 2,262,044Philpott Nov. 11, 1941 2,302,900 Vance Nov. 24, 1942

